Previously, the only general entry into this class of (aryl–O–
CH2CH2Rf) compounds is via the Mitsunobu reaction and the
yield is far from satisfactory.9 The limitations of this exchange
protocol were also probed. Unfortunately, only primary alcohols
are good substrates for this reaction. Attempts to use menthol,
cholesterol, and cyclohexanol in the reaction produced complex
mixtures. Alcohols that are easily ionized (allyl and benzyl alcohol)
preferentially alkylate toluene under the reaction conditions. The
reaction with 4-nitrobenzyl alcohol proceeded in low yield. The
reaction with 1-adamantanol is also sluggish and complicated.
The mechanism of the reaction became obvious once the superb
proton affinity of anthracene was recognized.10 As shown in
Scheme 2, protonation of anthracene at the 9 (or 1) position gives
the intermediate cationic species. The activation of the carbon–
oxygen bond was achieved through the strong electron-with-
drawing nature of the cation which was trapped by the excess
alcohol at the ipso position to produce the mixed-ketal
intermediate. Elimination of the methanol molecule regenerated
the aromaticity and furnished the desired exchange product.
As demonstrated in table 2, isomeric dimethoxyanthracene
derivatives all underwent efficient double exchange reaction to
furnish the expected products (table 2, entries 1–6). To react 2,
3-dimethoxyanthracene (4) and oligoethylene glycol seems to be an
appealing route to anthracene annulated crown ethers. However,
when 4 and 1.5 equivalents of triethylene glycol were put under the
Table 3 Multiple and selective ether–ether exchange reactions
reaction conditions (entry 7), only 4c was isolated. Evidently, the
lone pair on the ether oxygen is nucleophilic enough to undergo
intramolecular cyclization with the protonated anthracene. In fact,
4c is also observed as the minor product in entry 5 and 6 (25 and
5%). Nevertheless, both 4d and 4e are now easily accessible and
can be coverted into crown ethers in one simple step.
The exchange reaction can also be applied to the synthesis of
multiple ether substituted anthracenes without noticeable dete-
rioration of product yield. Our preliminary results into this venue
are shown in Table 3. The efficiency of these multi-sited reactions
might be due to the superior proton affinitiy of 5 and 6. An
interesting chemo-selectivity was observed in entry 3. Only the
methoxy groups directly attached to the anthracene core can be
replaced. Even when we use 8 equivalents of 1-octanol, the anisyl
methoxy group remains intact. This disparity in reactivity is most
likely due to the fact that the anthracene plane and anisyl groups
are orthogonal to each other. As a result, the protonated
anthracene is unable to activate the methoxy groups on the anisyl
substituents.
Scheme 2 Postulated mechanism for the ether–ether exchange reaction.
Table 2 Double exchange reaction in dimethoxyanthracene derivatives
In summary, we have developed a convenient protocol to make
various anthracene ethers via exchange reaction. The reaction
conditions are compatible with several common functional groups.
Currently, we are trying to extend this reaction to higher acenes
and other group VI elements.11
Chih-Hsiu Lin* and Krishnan Radhakrishnan{
Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan, Republic
of China. E-mail: chl@chem.sinica.edu.tw; Fax: +886 22 2783 1237;
Tel: +886 22 789 8540
Notes and references
1 S. Kilvelson and O. L. Chapman, Phys. Rev. Lett. B., 1983, 28, 7236;
M. Kertesz and R. Hoffmann, Solid State Commun., 1983, 47,
97; K. N. Houk, P. S. Lee and M. Nendel, J Org. Chem., 2001, 66,
5517; M. Bendikov, H. M. Duong, K. Starkey, K. N. Houk,
E. A. Carter and F. Wudl, J. Am. Chem. Soc., 2004, 126, 7416.
2 Y.-Y. Lin, D. J. Gundlach, S. Nelson and T. N. Jackson, IEEE Trans.
Electron Devices, 1997, 44, 1325; Y. Sakamoto, T. Suzuki,
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 504–506 | 505
Lin, Chih-Hsiu
